We propose a new approach to signal amplification for ultrasensitive, high-resolution localization of targets in cells and tissues. Amplification will be conducted using reiterative cycles of discrete, highly specific chemical reactions, each of which introduces a controlled number of labels and a consistent number of new binding sites for the next cycle. Because each cycle is initiated by the addition of reagent, the number of cycles, and therefore the resulting degree of amplification and detection sensitivity, can be programmed for optimum performance in the system under study. The chemical reactions will be much less subject to batch variations than enzymatic activity, yielding improvements in reproducibility which will accelerate the development and clinical validation of new molecular diagnostics such as in situ hybridization assays. Three interactions will be used: (i) chelation using polyhisitidine / nitrilotriacetic acid (NTA) - nickel (II) chelate reagent pairs, (ii) complementary oligonucleotide hybridization, and (iii) noncovalent ion pairing of ionized cationic and anionic reagent pairs. The detection label of choice for the new system will be the 1 4 nm Nanogold gold particle, combined with autometallic enhancement to generate a visible signal for interpretation by light microscopy or by eye, while enabling electron microscope localization if necessary.